1. Field of the Invention
The present invention relates to image reading apparatuses, and more particularly, to an image reading apparatus reading an image by a color image sensor, and processing the same.
2. Description of the Related Art
An image reading apparatus reading a stationary image of a color original by an image sensor to provide an image signal after subjecting read image data to various processing has been conventionally used as image input means of a computer and image input means of a digital copying machine.
In a full color image reading apparatus, a light source having a large quantity of light such as a tungsten halogen lamp is often used. Light reflected from the original by the light source is separated into its spectral components of three primary colors through each transmission filter of R, G, B, to form an image on a monochromatic CCD (image sensor). Depending on how to separate light into its spectral components of three primary colors, there are various systems such as a filter switching system, an on-chip filter system and the like.
In order to cut a near infrared light components of a long wavelength of 700 nm or more, which causes noise, out of various light components reflected from the original, an interference film filter is generally inserted in a light path.
With popularization of color image processing, there are many cases in which a number of color originals are read continuously. With reading of the original made higher in speed, a quantity of light of the light source is increased. Therefore, temperatures of the image sensor, various filters and the like are easy to be elevated by heat discharged from the light source.
Rise of the temperature of the filters causes a change of spectral characteristics of the filters, whereby the level of quantity of light of each color light incident on the image sensor is altered. For example, in the inference film filter, moisture in the filter is evaporated by rise of the temperature, causing the transmission range to be shifted to the short wavelength to lower the transmittance of the red color light range. Therefore, as shown by a curve CL2 of FIG. 8, the level of quantity of light of the red color light incident on the CCD is lowered. Rise of the temperature of the CCD also changes its sensitiveness characteristics or output characteristics.
Consequently, color reproductivity of the image data provided from the image reading apparatus is degraded, causing a problem, for example, in a color copying machine or the like, that the chromaticity of the copied image is different from that of the original, and that the picture quality is degraded. Therefore, it is necessary to interrupt reading of the original data to wait for lowering of the temperature, whereby the operation efficiency has been substantially decreased.